1. Field of the Invention
This invention relates to thermally insulated enclosures and cabinets and more particularly to thermally insulated cabinets for protection against cyclic thermal exposure.
2. Description of the Related Art
Heat sensitive materials and components must sometimes be located in an environment that experiences heat fluctuations that are detrimental to their longevity or effectiveness. For example, the telecommunications industry typically uses lead acid batteries as a source of back-up power in the event of a power failure in locations removed from environmentally protected buildings. The batteries are typically stored in a cabinet that is exposed to elevated temperature from daytime ambient (usually in the summer). Cabinets located in an outside environment can be exposed to temperatures ranging from minus 20° Celsius(C.) to plus 55° C., and the temperature experienced by the cabinet spreads into its interior. Lead acid batteries experience damage or shortened life-span when exposed to these extreme temperatures. High temperatures are especially damaging and the life expectancy of lead acid batteries can be reduced by half for every 10° C. above the optimum 25° C. that the batteries are stored in.
The most common technique for protecting this type of equipment from extreme temperatures is to use an electric heater to raise low temperatures in the cabinet and air conditioning to lower high temperatures in the cabinet. These types of cabinets, however, are relatively expensive and the operating power for the heater and air conditioner adds to the overall operating expense of the cabinet. The heater and air conditioning units also require periodic maintenance of filters and compressors, and repair when the units fail. This also adds operating expense to the cabinets.
An apparatus for protecting batteries against damaging heat is disclosed in U.S. Pat. No. 6,006,944 to C. Machledt. A storage vault is described for storing batteries underground. Through the use of an underground vault, the sides are not exposed to ambient air temperatures and the sun. The top is insulated with fiberglass to protect the batteries from overhead sun. One disadvantage of such an underground vault is the high cost of installation, necessitating excavation for installation and the complexities of building a structure underground. Another disadvantage is the added cost of maintenance procedures that must be conducted underground.
Another apparatus for protecting either materials or components is disclosed in U.S. Pat. No. 5,832,988 to Mistry, which utilizes heat exchangers to cool cabinets. A columnar heat exchanger is mounted within the cabinet and is used in combination with a diverter for controlling the flow of outdoor air through the system. One disadvantage of this apparatus is that lead acid batteries do not generate much heat when idle or in operation, and heat exchangers typically only work when there is a temperature difference between the inside and outside environments with energy moving in the direction of higher temperature to lower temperature.